It is known to introduce a pulverulent, granular or other finely divided solid absorbent into a furnace exhaust gas system, e.g. a furnace exhaust gas generated in the operation of a water boiler and especially the flue gases of a power plant boiler, to absorb sulfur dioxide and other toxic and noxious components of the flue gas.
In the dry process for removal of such components, the particulate absorbent is brought into contact with the flue gas for a period sufficient to enable absorption and even reaction of the sulfur dioxide with the absorbent, whereupon the solids are separated from the remaining portions of the flue gas by dry separation techniques.
Dry separation techniques include direction-change, impingement baffle, velocity-change and centrifugal separators as well as particle-trapping filters and electrostatic precipitators.
The absorbents hithereto used in the removal of sulfur dioxide from power plant boiler flue gases are generally finely divided inorganic alkaline-earth compounds especially calcium and/or magnesium compounds such as the calcium and/or magnesium oxides, hydroxides or carbonates. When reference is made to an absorbent compound, therefore, these compounds and especially the calcium compounds and particularly calcium carbonate are intended.
It is known to improve the absorption interaction of the particulate absorbent with the flue gas by including additives in the absorbent in the form of the halogen acids and halide salts. This approach, however, is disadvantageous in some cases because of the danger of release into the atmosphere of the additives or components thereof.
Nonetheless such additives are used because they do improve the reactivity of the absorbent.
Mention should also be made of the fact that there are numerous processes for the cleaning of flue gases from fossil-fuel fired boilers. The particular absorbent used will depend in each case upon the nature of the reaction desired with the components to be removed from the flue gases.
In general, one can distinguish between the wet and dry processes.
In the wet process, the gas is treated with a liquid, i.e. is scrubbed with a scrubbing solution, the liquid containing the absorbent in solution or in suspension.
In the dry process, however, the absorbent is introduced during the firing or immediately thereafter and, since the absorbent can be present in the combustion chamber, it may be added to the fuel (U.S. Pat. No. 4,262,610).
An advantage of the latter procedure is that the absorption commences as soon as the noxious or toxic compounds are formed.
The wet processes have the advantage of high efficiency, since the scrubbing operation contributes to the removal of the toxic and noxious compounds beyond the absorption reaction, although these procedures have the disadvantage of high capital cost, greater maintenance requirements for the equipment and an undesired cooling of the flue gases which may require them to be reheated if further processing is necessary or desired. Consequently, in many instances, the dry process is preferred.
The dry processes used heretofore, however, have been found to have relatively low efficiency, especially when applied to the removal of sulfur dioxide and other noxious or toxic components from the flue gases of power plant boilers. The poor efficiency appears to be a result of the relatively short contact time between the absorbent and the flue gases, even when the absorbent is present in the fuel and the contact time begins immediately upon combustion.
For a sufficient degree of removal of the toxic and noxious components, therefore, comparatively large quantities of absorbent must be used and this, of course, results in high operating cost.
Naturally it is important not to overload the combustion operation with excessive absorbent.
Consequently, operations heretofore in the dry process have had to balance the desire to avoid emissions to the environment which might be detrimental with the need to avoid destructive action with respect to the combustion, and with the need to ensure sufficient or satisfactory removal of the noxious and toxic components from the flue gases.